The present invention relates to crosslinked (reticulated) polyethylene compositions and processes for obtaining them.
Many different grades of polyethylene are manufactured for different applications, and equally there is a wide variety of physical properties of polyethylene which are important in each case. Crosslinked polyethylene is polyethylene in which the polymer chains are connected by covalent bonding to form a three-dimensional network. Such polyethylene generally has a range of enhanced properties compared with conventional polyethylene: improved heat stability, better chemical and stress crack resistance at high temperature, increased impact strength and abrasion resistance, reduced shrinkage, and enhanced long-term strength.
The use of certain transition metal compounds to polymerise 1-olefins, for example, ethylene, is well established in the prior art. Silica-supported chromium catalysts using the Phillips process have been known for several decades. The use of Ziegler-Natta catalysts, for example those catalysts produced by activating titanium halides with organometallic compounds such as triethylaluminium, is fundamental to many commercial processes for manufacturing polyolefins. In recent years the use of certain metallocene catalysts (for example biscyclopentadienylzirconiumdichloride activated with alumoxane) has provided catalysts with potentially high activity. These different catalyst systems provide polymeric products with a variety of properties.
Commodity polyethylenes are produced commercially in a variety of-different types and grades. Homopolymerisation of ethylene with transition metal based catalysts leads to the production of so-called “high density” grades of polyethylene. These polymers have relatively high stiffness and are useful for making articles where inherent rigidity is required, such as pipe and moulded products. Such articles are particularly suited to the use of crosslinked polyethylene.
WO 99/12981 discloses that ethylene may be polymerised using catalysts based on certain complexes, normally iron or cobalt, of selected 2,6-pyridinecarboxaldehydebis(imines) and 2,6-diacylpyridinebis(imines). Examples of high-density polyethylene compositions produced using such catalysts are disclosed in WO 99/46308. In WO 00/15646, which discloses specific complexes in the same class, and also in WO 00/24788, which discloses specific catalyst systems within the same class, it is mentioned generally that polymers of ethylene and propylene may be crosslinked using peroxide.